1. Field of the Invention
This invention relates generally to apparatus, such as a video tape recorder (VTR) of the so-called "helical-scan type", in which video or other information signals are recorded in successive parallel tracks which are skewed or extend obliquely on a magnetic tape and, more particularly, is directed to an improved tracking control system by which a magnetic head or other transducer in such apparatus is made to accurately scan the track or tracks in which the video or other information signals are recorded.
2. Description of the Prior Art
In a helical-scan VTR, the magnetic tape extends helically about at least a portion of the periphery of a guide drum and is adapted to be moved or advanced in the longitudinal direction of the tape while at least a portion of the guide drum is rotated, and the transducer or magnetic head is mounted on a rotated portion of the guide drum so as to rotate with the latter and thereby repeatedly scan across the tape in a path at an angle to the longitudinal direction of the tape. During the recording operation of the VTR, the angle between the scanning path, and hence each record track, and the longitudinal direction of the tape is dependent on the rotational speed of the rotary head and also the speed at which the magnetic tape is longitudinally advanced. Accordingly, if the speed and direction of advancement of the magnetic tape are not the same during the reproducing operation as during the recording operation, then the scanning path of the magnetic head during reproducing will not precisely follow or coincide with a record track or the tape during each movement of the head across the tape and, accordingly, the recorded video or other information signals may not be correctly or accurately reproduced.
Various tracking control or servo systems have been proposed for maintaining correct tracking or scanning of the record tracks by the rotary head. In the most desirable of these known arrangements, means are provided for deflecting the head in a direction normal to the plane of its rotation, that is, in a direction which is transverse in respect to the direction along each of the record tracks, and the amount of such deflection is electrically controlled during the movement of the head along each track so as to achieve accurate scanning of the latter. In published Japanese Patent Application No. 9919/1974 (Applicant: Matsushita Electric Industrial Company, Limited), it is generally disclosed to control the amount of deflection of the head in the direction normal to its plane of rotation in dependence on the difference between the speeds of advancement of the tape during the recording and reproducing operations so as to theoretically make possible the correct reproduction of the recorded video signals in the still-motion made in which the tape is at rest, in the slow-motion mode in which the speed of advancement of the tape is, for example, 1/4 or 1/2 the tape speed for the recording operation, and in the quick-motion mode in which the speed of advancement of the tape is substantially greater than, for example, 2 times, the tape speed, for the recording operation. Further, in published Japanese Patent Application No. 117,106/1977 (Applicant: Ampex Corporation), it is disclosed to detect the amount of deflection of the head required for accurately scanning a record track thereby, and, when the detected deflection approaches the physical limit of that possible with the transducer deflecting device supporting the head, the electrical drive signal for the transducer deflecting device is controlled to cause the head to scan the next adjacent record track, that is, to effect a so-called "track jump". By reason of such "track jump" it is theoretically possible to effect correct slow-motion reproduction and also reverse-motion reproduction in which, for example, the tape is advanced longitudinally at the same speed as for the recording operation, but in the reverse or opposite direction.
However, in many existing tracking control or servo systems, the amplitude of the deflection of the rotary head or transducer by its transducer deflecting device is not optimized, that is, the maximum required deflection of the head in the non-normal reproduction mode, for example, the still-motion, slow-motion, fast-forward and reverse-motion modes, is not minimized. In U.S. Patent Applications Ser. Nos. 06/073,246, now U.S. Pat. No. 4,287,538, 06/152,117, and 06/232,014, filed Sept. 6, 1979, May 21, 1980, and Feb. 6, 1981, respectively, all having a common assignee herewith, there is disclosed an automatic tracking control system which optimizes the amplitude of the deflection of the rotary head by its deflecting device for each of the non-normal reproducing modes. More particularly, in the foregoing automatic tracking control systems, the drive signal applied to the transducer deflecting device comprises a jump signal or track selection control signal for determining the starting position of the transducer or head and, thereby, the next desired track to be followed or scanned thereby, a dithering signal or oscillation, a track following error signal, and a slant angle correction signal. In this manner, the tracking error between the recorded track on the magnetic tape and the scanning path of the reproducing head is corrected to effect coincidence therebetween. A reproduced picture of high quality without any guard band noise can thus be obtained even when the tape speed is varied during various special reproducing modes, such as during slow-, still-, fast- or reverse motion-playback modes.
Generally, the transducer deflecting device in the above systems is constituted by an electro-mechanical element, such as a bi-morph leaf, which is constructed by laminating two piezo-ceramic leaves with at least one electrode plate, and which is deflected in accordance with a drive voltage supplied thereto. However, the range within which such bi-morph leaf can be linearly deflected is limited. This, of course, limits the range of the drive voltage that can be applied thereto, for example, in the range of .+-.200 V, to obtain such linear deflection. Since the amount of deflection that is necessary for correct scanning of the record tracks, is generally greater for higher tape speeds, the aforementioned linear deflection limitation consequently results in a limitation of the reproducing speed of the tape, such as in the fast-forward and reverse reproducing modes. For example, when the allowable range of the drive voltage applied to the bi-morph leaf is .+-.200 V, reproduction at speeds greater than -0.5 times the normal reproducing speed (in the reverse direction), that is, during slow-motion reverse playback, and greater than +2.5 times the normal reproducing speed (in the forward direction), that is, during fast-foward playback, cannot be accomplished without substantial deterioration of the reproduced picture.